1. Field of the Invention
The present invention relates to a method of and apparatus for determining the temperature at which a substance changes state and is particularly, although not exclusively, concerned with determining the melting point of a substance.
2. DESCRIPTION OF THE RELATED ART
A prior method of determining the melting point of a substance comprises emitting light from a source and directing that light at a solid substance. A photocell is located on the other side of the substance to receive any light transmitted through the substance. The substance is then heated. Some melted substances allow more light to be transmitted therethrough when they are melted and the increase in light transmitted through the substance indicates the melting point temperature of the substance. However, some substances become frothy or are dark when they melt and thus there may be no light, or no change in light received by the photocell, or no such discernible change when the substance melts thereby rendering determination of the melting point impossible with this method. Furthermore, the substance is placed in a capillary tube prior to having light directed at it which is a difficult and messy operation to perform.
An alternative prior method of determining the melting point of a substance is shown in GB 2 202 941A (Fisons) in which light is emitted downwardly on to a solid sample which is being heated. When the sample becomes molten, light is no longer scattered by the sample but is instead reflected upwardly, and the change determined by the detector is used to indicate that the sample has melted. However, as the temperature of the heating block is continually rising, and as there is a delay between the time when the bottom of the sample melts and the time when the top surface melts, by the time the change in reflection is determined the temperature of the heating block will have risen slightly and the recorded melting temperature will not be the actual melting temperature. Furthermore, the location of the light source and light detector above the sample plate causes the source and detector to be in the way when the plate is loaded and unloaded, and the source and detector are exposed to the environment and any splashes or vapour rising from the sample thereby becoming dirty and impairing their function.